The present invention relates to an electrophotographic copying process; and more particularly to such a process for obtaining paper copies of documents recorded on microfilm.
Electrophotographic processes for producing a permanent image on plain paper are well known and commonly used in office copiers, laser printers and microfilm viewer/printers. In general, all these processes include: (1) charging a photoreceptor which is a drum or continuous belt bearing a photoconductive material; (2) exposing the charged area to a light image to produce an electrostatic charge image; (3) applying developer particles (toner) to the photoreceptor surface bearing the image to form a visible image; (4) transferring the particles in the form of the image from the photoreceptor to paper; (5) fusing or fixing the transferred particles to the paper; and (6) cleaning or restoring the photoreceptor for the next printing cycle. Further information about electrophotographic processes is available in the text "The Physics and Technology of Xerographic Processes", by Edgar M. Williams, 1984, a Wiley-Interscience Publication of John Wiley & Sons, the disclosure of which is hereby incorporated by reference.
Due to the curved surface of a photoreceptor drum, an image must be formed in sections on that surface by scanning the object being reproduced. The object scanning has to be synchronized with the movement of the drum, thereby adding to the complexity of the copier. The complexity rises even more when image scan projection is employed in a microfilm viewer/printer. Although the magnification ratio for copy machines typically is no greater that 2 to 1, the magnification required in microfilm viewer/printer ranges from 12 to 1 all the way up to 72 to 1. This increased magnification necessitates extremely consistent movement of the scanning system to avoid blurring the reproduced image. In addition, a scanning system increases the size of both types of reproduction apparatus.
An alternative approach to image scanning, referred to as "full plane exposure," projects the entire image onto a portion of the photoreceptor belt surface that has been stretched flat. U.S. Pat. No. 4,616,922 describes a microfilm viewer/printer that uses full plane exposure of a photoreceptor belt. That system addresses the problems associated with image scanning, but does not adequately address the problem of apparatus size. The loop of the photoreceptor belt has to be long enough so that the individual devices that perform separate steps of the copying process can be located around the perimeter of the belt. Furthermore enough clear space must be provided between the charging and the developing devices as well as between the developing and transfer devices so that a portion of the belt containing an image can be parked in both those locations during the processing.
While it is desirable to utilize full plane photoreceptor exposure in electrophotographic equipment, reducing the size of such equipment still needs to be addressed.